The present invention relates to the art of making aluminum base alloy extruded products, and is particularly concerned with extruded products which receive a homogenization heat treatment prior to extrusion.
The metal working process known as extrusion involves pressing metal stock through a die opening of predetermined configuration in order to form a shape of indefinite length and substantially constant cross section. In the die extrusion process, with which this invention is concerned, the preheated aluminum base alloy stock is placed in a cylinder, usually heated, having a suitable die at one end and a reciprocable piston or ram of approximately the same cross sectional dimensions as the bore of the cylinder. The piston or ram moves against the stock to compress the stock and cause the metal to flow through the die opening. The pressure exerted on the stock during the operation raises the internal temperature of the stock as a result of internal friction within the metal body.
The present invention is particularly concerned with aluminum alloys of the aluminum-magnesium-silicon type. Extruded profiles of aluminum-magnesium-silicon alloys have considerable commercial value. When heat hardened, such profiles have desirably high strength characteristics. In order to produce such profiles in the most economical manner extrusion should be carried out at the highest speed possible. Conventionally, the extrudability of these alloys is improved by subjecting the cast ingot to an elevated temperature homogenizing process, such as at 955.degree. - 1025.degree. F for from 4 to 12 hours followed by air cooling. It is naturally highly desirable to provide a process for economically improving extrusion speed while maintaining desirable product characteristics.
However, extrusion speed is a factor which affects the quality of an extruded product. In order to achieve acceptable surface quality a certain range of extrusion speeds must be observed, with the range being related to the extrusion size and the reduction in cross sectional area effected by the extrusion. Exceeding the predetermined speed generally causes a rupture of the surface and also other defects which result in rejection of the product.
A limiting factor for extrusion of an aluminum alloy is the onset at some extrusion rate of the phenomenon known as surface checking or chatter cracks. These are surface defects which form a pattern of fine transverse cracks resulting from longitudinal tensile stresses which are high compared with the strength of the alloy at its working temperature. Incipient cracks may be no deeper than 0.001 to 0.005 inch; however, they are unacceptable from the standpoint of surface appearance, finishing ability, dimensional accuracy and mechanical integrity. It is known that the surface checking phenomenon occurs at lower speeds as the extrusion temperature is raised. In addition, high strength alloys must be extruded more slowly and at lower temperatures in order to avoid cracking. This suggests that there is a relationship between flow stresses and cracking tendency due to rises in extrusion surface temperature caused by adiabatic heating.